The formation and development of an image on the surface of a photoconductive member by electrophotographic means is well known in the art. Basically, the technique involves placing a uniform electrostatic charge on the surface, exposing the charged surface to a pattern of light so as to form thereon a latent electrostatic image and then developing the latent electrostatic image by depositing on the surface finely divided electroscopic material referred to in the art as "toner." The toner is attracted to those areas of the surface retaining the electrostatic charge, thereby forming a toned image. The toned image may either be fixed to the surface of the photoconductive member by heat lamps or other suitable means or transferred to a secondary support surface such as paper and then fixed if desired or necessary.
In some known electrophotographic copying or duplicating machines the photoconductive member is in the form of a drum which rotates in unison relative to a plurality of processing stations. For high speed copying, however, it has been found necessary that the photoconductive surface be in a flattened condition at the time of exposure in order to insure complete focusing of the original being copied. Consequently, it has been found advantageous to employ a photoconductive member in the form of an endless belt or web mounted for rotational movement on a plurality of rollers.
Regardless of whether the photoconductive member is in the form of a drum or a belt mounted on rollers, the latent electrostatic image so formed can be developed into a visible image by using any one of several known techniques; these include cascade development, magnetic brush development and liquid development. In liquid development a dispersion of electroscopic particles in an insulating liquid is employed and the electrostatic image developed by deposition of particles from the liquid to the photoconductive surface. In such development the liquid containing the particles contacts the photoconductive surface in both the charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern, the suspended particles migrate toward the charged portions of the surface separating out of the insulating liquid. The electrophoretic migration of charged particles results in the deposition of charged particles on the imaging surface in an image configuration. Such development has been obtained in the past by flowing the liquid developer over the image bearing surface, by immersing the image bearing surface in a bath of the developer liquid and by presenting the developer liquid on a smooth surfaced roller and moving the roller against the imaging surface. In connection with these various techniques, it is known that the development can be improved, especially if the image contains large solid areas, through the use of what is known in the art as a development electrode.
In U.S. Pat. No. 4,025,339 issued on May 24, 1977 to M. R. Kuehnle there is described an electrophotographic film that is capable of being imaged with quality and gray scale, as good as, if not better than, that achieved by photographic techniques. The film comprises an inorganic coating of microcrystalline material that is bonded onto a conductive substrate. The inorganic coating may comprise a layer of about 2,000 Angstroms to 2 microns thick of radio frequency sputtered cadmium sulfide. The conductive substrate may comprise a layer of about 500 Angstroms thick of indium tin oxide on a sheet of stable polyester plastic about 5 microns thick. A latent electrostatic image formed on the film may be developed using a liquid toner.
In order to make the fullest use of the exceptional properties of the electrophotographic film described in the above noted patent, especially for high speed duplicating or copying machine applications, there is a need for a simple yet efficient technique for developing a latent electrostatic image formed thereon using a liquid toner.